H2O and HCl adsorption on single crystal α-Al2O3(0001) at stratospheric temperatures

Abstract The adsorption of H2O and HCl on single crystal α-Al2O3(0001) at stratospheric temperatures was studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The α-Al2O3(0001) surface was used to model stratospheric Al2O3 exhaust particles from solid rocket motors. H2O and HCl coverages may influence heterogeneous chemistry on Al2O3 particles at stratospheric temperatures of ∼200–270 K. Compared with earlier measurements at 298–300 K, the saturation coverages of both H2O and HCl on α-Al2O3(0001) were approximately two times higher at 210 K. The saturation coverages of H2O and HCl at 210 K were ΘH2O=4.4×1014 molecules/cm2 and ΘHCl=2.0×1014 molecules/cm2. The sticking coefficients for both H2O and HCl on α-Al2O3(0001) at 210 K were also higher than the corresponding sticking coefficients at 298–300 K. The sticking coefficients (S) for both H2O and HCl at 210 K were ∼0.08 at the lowest coverages. The TPD spectra revealed that H2O and HCl desorption occurred over a broad range of temperatures. These results indicate that H2O and HCl adsorb onto a wide distribution of binding sites on the α-Al2O3(0001) surface. The LITD and TPD results predict that α-Al2O3 rocket exhaust particles will be covered with chemisorbed H2O and HCl under stratospheric conditions. Additional H2O and HCl exposure can also replace the adsorbed HCl and H2O species, respectively, by exchange reactions.